Prior to the present invention, it has been known to make drums as large as 55 gallons by blow-molding, and to employ molds therefor which create flat tops. Such flat tops are acceptacle in the trade because of the traditional shape of metal drums; in addition to the relative ease of design, they permit the ready application of labels, stencils and the like which are necessary to commercial usage.
Blow-molding, however, is not conducive to use in manufacturing containers with sharp corners unless the parison is generated with excesses of thermoplastic material for areas destined to be expanded into the sharp corners. The generation of such irregular parisons is difficult even with computerized controls and frequently results in excess material where it is not wanted or needed. A flat-top 55-gallon drum will of course have a rather sharp corner for the parison to occupy around the entire periphery of the top. Tyipcally, the top chime, representing the intersection of the circular surface of the top of the drum and the cylindrical surface of the body, will be designed as a curved surface having a radius of about one-half inch. This means that the parison must expand outwardly to a point wherein its wall thickness will be significantly reduced at a point where strength is desired. Attempts to alleviate the problem somewhat by employing a slightly protruding spherical surface have not solved the problem and have proven troublesome because of the difficulty of applying labels and stencils to a spherical surface.